Cool! I had those when I lived up north too! You could plant almost any fall flower as most don't need a lot [if any] sun. Mums come to mind and they are plentiful [for example, you can buy them at the local Jewel] You might also consider a fall ivy or a cold-weather moss which can look like a nice little velvet carpet; just to add some color. The other thing might be to do a colorful art glass installation. Just line up different colored and differently-shaped bottles [careful not to make it look like leftovers from a party]. You probably aren't allowed, but it would be fun to paint the doors different colors, or to paint an image of a flower box with fully-bloomed flowers.
Just a few ideas! Good luck!

Yes mums are very good, also are those decorative heads of cabbage. they are gorgeous.

what about mini pine trees?